Quantification of thoracic volumes by three-dimensional imaging.

End-expiratory thoracic cavity volume (Vthx) was measured in eight volunteers lying supine by three-dimensional X-ray computed tomography using the Dynamic Spatial Reconstructor. Untrapped end-expiratory pulmonary gas volume at functional residual capacity (FRC) was determined by nitrogen clearance. Both measurements were done before and after induction of anesthesia-paralysis. After induction of anesthesia-paralysis, Vthx and FRC were consistently and significantly (P less than 0.01) reduced by 0.28 +/- 0.22 (SD) and 0.59 +/- 0.24 liter, respectively. The reduction of FRC was larger than the reduction of Vthx (delta Vthx) in six of the eight subjects, a finding suggesting that intrathoracic fluid (blood) plus trapped gas volume (Vtt) increased. Changes in Vthx were partitioned into volume changes from the thoracic rib cage (delta Vrc) and from shape and/or position changes of the diaphragm (delta Vdi). delta Vrc contributed significantly (0.17 +/- 0.15 liter, P less than 0.02) to delta Vthx, whereas delta Vdi contributed only in four of the eight subjects. We conclude that delta Vrc, delta Vdi, and delta Vtt contribute to the reduction of FRC after induction of anesthesia-paralysis in humans; the relative contribution of them varies among subjects.